FAQ

This is the list of Frequently Asked Questions about Chicken Scheme. If you have a question not answered here, feel free to post to the chicken-users mailing list; if you consider your question general enough, feel free to add it to this list.

General

Why yet another Scheme implementation?

Since Scheme is a relatively simple language, a large number of implementations exist and each has its specific advantages and disadvantages. Some are fast, some provide a rich programming environment. Some are free, others are tailored to specific domains, and so on. The reasons for the existence of CHICKEN are:

CHICKEN is portable because it generates C code that runs on a large number of platforms.

CHICKEN is extendable, since its code generation scheme and runtime system/garbage collector fits neatly into a C environment.

CHICKEN is free and can be freely distributed, including its source code.

CHICKEN offers better performance than nearly all interpreter based implementations, but still provides full Scheme semantics.

As far as we know, CHICKEN is the first implementation of Scheme that uses Henry Baker's Cheney on the M.T.A concept.

What should I do if I find a bug?

Send e-mail to felix@call-with-current-continuation.org with some hints about the problem, like version/build of the compiler, platform, system configuration, code that causes the bug, etc.

Specific

Why are values defined with define-foreign-variable or define-constant or define-inline not seen outside of the containing source file?

Accesses to foreign variables are translated directly into C constructs that access the variable, so the Scheme name given to that variable does only exist during compile-time. The same goes for constant- and inline-definitions: The name is only there to tell the compiler that this reference is to be replaced with the actual value.

How does cond-expand know which features are registered in used units?

Each unit used via (declare (uses ...)) is registered as a feature and so a symbol with the unit-name can be tested by cond-expand during macro-expansion-time. Features registered using the register-feature! procedure are only available during run-time of the compiled file. You can use the eval-when form to register features at compile time.

Why are constants defined by define-constant not honoured in case constructs?

case expands into a cascaded if expression, where the first item in each arm is treated as a quoted list. So the case macro can not infer whether a symbol is to be treated as a constant-name (defined via define-constant) or a literal symbol.

How can I enable case sensitive reading/writing in user code?

To enable the read procedure to read symbols and identifiers case sensitive, you can set the parameter case-sensitivity to #t.

Why doesn't CHICKEN support the full numeric tower by default?

The short answer:

% chicken-install numbers
% csi -q
#;1> (use numbers)

The long answer:

There are a number of reasons for this:

- For most applications of Scheme fixnums (exact word-sized integers) and flonums (64-bit floating-point numbers) are more than sufficient;

- Interfacing to C is simpler;

- Dispatching of arithmetic operations is more efficient.

There is an extension based on the GNU Multiprecision Package that implements most of the full numeric tower, see numbers.

Does CHICKEN support native threads?

Native threads are not supported for two reasons. One, the runtime system is not reentrant. Two, concurrency implemented properly would require mandatory locking of every object that could be potentially shared between two threads. The garbage-collection algorithm would then become much more complex and inefficient, since the location of every object has to be accessed via a thread synchronization protocol. Such a design would make native threads in Chicken essentially equivalent to Unix processes and shared memory.

Does CHICKEN support Unicode strings?

The system does not directly support Unicode, but an extension for UTF8 strings exists for CHICKEN 3, which may be ported to the current CHICKEN version at some stage.

Why are `dynamic-wind' thunks not executed when a SRFI-18 thread signals an error?

Here is what Marc Feeley, the author of SRFI-18 has to say about this subject:

>No the default exception handler shouldn't invoke the after
> thunks of the current continuation. That's because the
> exception handler doesn't "continue" at the initial
> continuation of that thread. Here are the relevant words of
> SRFI 18:
>
> Moreover, in this dynamic environment the exception handler
> is bound to the "initial exception handler" which is a unary
> procedure which causes the (then) current thread to store in
> its end-exception field an "uncaught exception" object whose
> "reason" is the argument of the handler, abandon all mutexes
> it owns, and finally terminate.
>
>The rationale is that, when an uncaught exception occurs in a
>thread the thread is in bad shape and things have gone
>sufficiently wrong that there is no universally acceptable way to
>continue execution. Executing after thunks could require a
>whole lot of processing that the thread is not in a shape to do.
>So the safe thing is to terminate the thread. If the programmer
>knows how to recover from an exception, then he can capture the
>continuation early on, and install an exception handler which
>invokes the continuation. When the continuation is invoked the
>after thunks will execute.

Platform specific

How do I generate a DLL under MS Windows (tm) ?

Use csc in combination with the -dll option:

C:\> csc foo.scm -dll

How do I generate a GUI application under Windows(tm)?

Invoke csc with the -windows option. Or pass the -DC_WINDOWS_GUI option to the C compiler and link with the GUI version of the runtime system (that's libchicken-gui[-static].lib. The GUI runtime displays error messages in a message box and does some rudimentary command-line parsing.

Compiling very large files under Windows with the Microsoft C compiler fails with a message indicating insufficient heap space.

It seems that the Microsoft C compiler can only handle files up to a certain size, and it doesn't utilize virtual memory as well as the GNU C compiler, for example. Try closing running applications. If that fails, try to break up the Scheme code into several library units.

When I run csi inside an emacs buffer under Windows, nothing happens.

Invoke csi with the -:c runtime option. Under Windows the interpreter thinks it is not running under control of a terminal and doesn't print the prompt and does not flush the output stream properly.

I load compiled code dynamically in a Windows GUI application and it crashes.

Code compiled into a DLL to be loaded dynamically must be linked with the same runtime system as the loading application. That means that all dynamically loaded entities (including extensions built and installed with chicken-install) must be compiled with the -windowscsc option.

On Windows, csc.exe seems to be doing something wrong.

The Windows development tools include a C# compiler with the same name. Either invoke csc.exe with a full pathname, or put the directory where you installed CHICKEN in front of the MS development tool path in the PATH environment variable.

On Windows source and/or output filenames with embedded whitespace are not found.

There is no current workaround. Do not use filenames with embedded whitespace for code. However, command names with embedded whitespace will work correctly.

Customization

How do I run custom startup code before the runtime-system is invoked?

When you invoke the C compiler for your translated Scheme source program, add the C compiler option -DC_EMBEDDED, or pass -embedded to the csc driver program, so no entry-point function will be generated (main()). When your are finished with your startup processing, invoke:

CHICKEN_main(argc, argv, C_toplevel);

where C_toplevel is the entry-point into the compiled Scheme code. You should add the following declarations at the head of your code:

How can I add compiled user passes?

To add a compiled user pass instead of an interpreted one, create a library unit and recompile the main unit of the compiler (in the file chicken.scm) with an additional uses declaration. Then link all compiler modules and your (compiled) extension to create a new version of the compiler, like this (assuming all sources are in the current directory):

On platforms that support it (Linux ELF, Solaris, Windows + VC++), compiled code can be loaded via -extend just like source files (see load in the User's Manual).

Macros

Where is define-macro?

With CHICKEN 4, the macro-expansion subsystem is now hygienic where old Lisp-style low-level macros are not available anymore. define-syntax can define hygienic macros using syntax-rules or low-level macros with user-controlled hygienic with explicit renaming macros. Translating old-style macros into ER-macros isn't that hard, see Modules and macros for more information.

Macro bodies that are defined and used in a compiled source-file are evaluated during compilation and so have no access to anything created with define. Use define-for-syntax instead.

Why isn't load properly loading my library of macros?

During compile-time, macros are only available in the source file in which they are defined. Files included via include are considered part of the containing file.

Warnings and errors

Why does my program crash when I use callback functions (from Scheme to C and back to Scheme again)?

There are two reasons why code involving callbacks can crash out of no apparent reason:

It is important to use foreign-safe-lambda/foreign-safe-lambda* for the C code that is to call back into Scheme. If this is not done than sooner or later the available stack space will be exhausted.

If the C code uses a large amount of stack storage, or if Scheme-to-C-to-Scheme calls are nested deeply, then the available nursery space on the stack will run low. To avoid this it might be advisable to run the compiled code with a larger nursery setting, i.e. run the code with -:s... and a larger value than the default (for example -:s300k), or use the -nursery compiler option. Note that this can decrease runtime performance on some platforms.

Why does the linker complain about a missing function _C_..._toplevel?

This message indicates that your program uses a library-unit, but that the object-file or library was not supplied to the linker. If you have the unit foo, which is contained in foo.o than you have to supply it to the linker like this (assuming a GCC environment):

% csc program.scm foo.o -o program

Why does the linker complain about a missing function _C_toplevel?

This means you have compiled a library unit as an application. When a unit-declaration (as in (declare (unit ...))) is given, then this file has a specially named toplevel entry procedure. Just remove the declaration, or compile this file to an object-module and link it to your application code.

Why does my program crash when I compile a file with -unsafe or unsafe declarations?

The compiler option -unsafe or the declaration (declare (unsafe)) disable certain safety-checks to improve performance, so code that would normally trigger an error will work unexpectedly or even crash the running application. It is advisable to develop and debug a program in safe mode (without unsafe declarations) and use this feature only if the application works properly.

Why don't toplevel-continuations captured in interpreted code work?

Consider the following piece of code:

(define k (call-with-current-continuation (lambda(k) k)))(k k)

When compiled, this will loop endlessly. But when interpreted, (k k) will return to the read-eval-print loop! This happens because the continuation captured will eventually read the next toplevel expression from the standard-input (or an input-file if loading from a file). At the moment k was defined, the next expression was (k k). But when k is invoked, the next expression will be whatever follows after (k k). In other words, invoking a captured continuation will not rewind the file-position of the input source. A solution is to wrap the whole code into a (begin ...) expression, so all toplevel expressions will be loaded together.

Why does define-reader-ctor not work in my compiled program?

The problem is that the compiler reads the complete source-file before doing any processing on it, so the sharp-comma form is encountered before the reader-ctor is defined. A possible solution is to include the file containing the sharp-comma form, like this:

This typically happens because the Chicken libraries have been installed in a non-standard location, such as your home directory. The workaround is to explicitly tell the dynamic linker where to look for your libraries:

How can I increase the size of the trace shown when runtime errors are detected?

When a runtime error is detected, Chicken will print the last entries from the trace of functions called (unless your executable was compiled with the -no-trace option. By default, only 16 entries will be shown. To increase this number pass the -:aN parameter to your executable.

Optimizations

How can I obtain smaller executables?

If you don't need eval or the stuff in the extras library unit, you can just use the library unit:

(declare (uses library))(display "Hello, world!\n")

(Don't forget to compile with the -explicit-use option) Compiled with Visual C++ this generates an executable of around 240 kilobytes. It is theoretically possible to compile something without the library, but a program would have to implement quite a lot of support code on its own.

How can I obtain faster executables?

There are a number of declaration specifiers that should be used to speed up compiled files: declaring (standard-bindings) is mandatory, since this enables most optimizations. Even if some standard procedures should be redefined, you can list untouched bindings in the declaration. Declaring (extended-bindings) lets the compiler choose faster versions of certain internal library functions. This might give another speedup. You can also use the the usual-integrations declaration, which is identical to declaring standard-bindings and extended-bindings (note that usual-integrations is set by default). Declaring (block) tells the compiler that global procedures are not changed outside the current compilation unit, this gives the compiler some more opportunities for optimization. If no floating point arithmetic is required, then declaring (number-type fixnum) can give a big performance improvement, because the compiler can now inline most arithmetic operations. Declaring (unsafe) will switch off most safety checks. If threads are not used, you can declare (disable-interrupts). You should always use maximum optimizations settings for your C compiler. Good GCC compiler options on Pentium (and compatible) hardware are: -Os -fomit-frame-pointer -fno-strict-aliasing Some programs are very sensitive to the setting of the nursery (the first heap-generation). You should experiment with different nursery settings (either by compiling with the -nursery option or by using the -:s... runtime option).

Which non-standard procedures are treated specially when the extended-bindings or usual-integrations declaration or compiler option is used?

The following standard bindings are handled specially, depending on optimization options and compiler settings:

What's the difference betweem "block" and "local" mode?

In block mode, the compiler assumes that definitions in the current file are not visible from outside of the current compilation unit, so unused definitions can be removed and calls can be inlined. In local mode, definitions are not hidden, but the compiler assumes that they are not modified from other compilation units (or code evaluated at runtime), and thus allows inlining of them.

Can I load compiled code at runtime?

Yes. You can load compiled at code at runtime with load just as well as you can load Scheme source code. Compiled code will, of course, run faster.

To do this, pass to load a path for a shared object. Use a form such as (load "foo.so") and run csc -shared foo.scm to produce foo.so from foo.scm (at which point foo.scm will no longer be required).

Why is my program which uses regular expressions so slow?

The regular expression engine has recently be replaced by alex shinn's excellent irregex library, which is fully implemented in Scheme. Precompiling regular expressions to internal form is somewhat slower than with the old PCRE-based regex engine. It is advisable to use regexp to precompile regular expressions outside of time-critical loops and use them where performance matters.

Garbage collection

Why does a loop that doesn't cons still trigger garbage collections?

Under CHICKENs implementation policy, tail recursion is achieved simply by avoiding to return from a function call. Since the programs are CPS converted, a continuous sequence of nested procedure calls is performed. At some stage the stack-space has to run out and the current procedure and its parameters (including the current continuation) are stored somewhere in the runtime system. Now a minor garbage collection occurs and rescues all live data from the stack (the first heap generation) and moves it into the the second heap generation. Then the stack is cleared (using a longjmp) and execution can continue from the saved state. With this method arbitrary recursion (in tail- or non-tail position) can happen, provided the application doesn't run out of heap-space. (The difference between a tail- and a non-tail call is that the tail-call has no live data after it invokes its continuation - and so the amount of heap-space needed stays constant)

While you might expect objects to be reclaimed and "(1 2 3) is dead" printed, it won't happen: the literal list gets held in the interpreter history, because it is the result value of the set-finalizer! call. Running this in a normal program will work fine.

When testing finalizers from the interpreter, you might want to define a trivial macro such as

Users of *nix-like systems (including Cygwin), may also want to check out rlwrap. This program lets you "wrap" another process (e.g. rlwrap csi) with the readline library, giving you history, autocompletion, and the ability to set the keystroke set. Vi fans can get vi keystrokes by adding "set editing-mode vi" to their .inputrc file.

Does code loaded with load run compiled or interpreted?

If you compile a file with a call to load, the code will be loaded at runtime and, if the file loaded is a Scheme source code file (instead of a shared object), it will be interpreted (even if the caller program is compiled).

How do I use extended (non-standard) syntax in evaluated code at run-time?

Normally, only standard Scheme syntax is available to the evaluator. To use the extensions provided in the CHICKEN compiler and interpreter, add:

(require-library chicken-syntax)

Extensions

Where is "chicken-setup" ?

chicken-setup has been rewritten from scratch and its functionality is now contained in the three tools chicken-install, chicken-uninstall and chicken-status. See the Extensions chapter for more information.

How can I install Chicken eggs to a non-default location?

You can just set the CHICKEN_REPOSITORY environment variable. It should contain the path where you want eggs to be installed:

In order to make programs (including csi) see these eggs, you should set this variable when you run them. Alternatively, you can call the repository-path Scheme procedure before loading the eggs, as in:

(repository-path "/home/azul/chicken")(use format-modular)

Note, however, that using repository-path as above hard-codes the location of your eggs in your source files. While this might not be an issue in your case, it might be safe to keep this configuration outside of the source code (that is, specifying it as an environment variable) to make it easier to maintain.